bicycle-pedal mechanism (BP mechanism)

https://doi.org/10.1351/goldbook.BT07330
Volume-conserving mechanism proposed for the photoisomerization of conjugated double bonds. Two alternating double bonds rotate concertedly with only the two CH units turning in and out of the plane of the molecule.
BT07330.png
See also: hula-twist mechanism
Source:
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 308 (https://doi.org/10.1351/pac200779030293)